David -- the rule of thumb I have heard is that a slashdotting triples your hits for a couple of days, but in huge misshapen bursts than any kind of well-formed load. There's some sample bias here, in that generally a site featured in its own article on slashdot usually has substantial traffic already, a trend that has become more common as slashdot has evolved in the direction of a big circle jerk with its usual linked sites. A smaller site is harder to find numbers for, but it is increasingly unlikely that it will be slashdotted until said site has grown substantially and has been putting out neat stuff for some time...
As an aside, I think 2 years ago a prof somewhere contacted a number of /.ed sites and graphed the common pattern of hits following the /. post -- and when his paper was /.ed, his site matched his numbers. If you grovel around in the /. search pages you might find it.
